III-V films, such as Al-Ga-As films, useful for semiconducting applications, may be grown by chemical vapor deposition (CVD) as described by R. Bhat et al. J. of Crystal Growth, 77 (1986) 7-10. CVD requires a decomposable source material of each of the metallic elements to be deposited. As Bhat et al. describe, the decomposable source of aluminum has traditionally been trimethylaluminum; however, dimethylaluminum hydride is advantageous in that less carbon contamination in the film results.
Dimethylaluminum hydride (DMAH) is conventionally prepared from lithium aluminum hydride (LiAlH.sub.4) (referred to herein as LAH) and trimethylaluminum (TMA) by reaction (I) as follows: EQU (CH.sub.3).sub.3 Al+LiAlH.sub.4 .fwdarw.(CH.sub.3).sub.2 AlH+LiAlH.sub.3 CH.sub.3,(I)
L T. Wartick et al. J.A.C.S. 75 (1953) 835. It has also been prepared by splitting TMA under hydrogen pressure at 150-200.degree. C.; T. Koster et al., German Democratic Republic Patent 16650 (1957); H.E. Podall et al. J. Org. Chem. 24 (1959) 1222.
Herein, is described a method of producing DMAH from the waste products of the production of a decomposable chemical, trimethylgallium (TMG), which is used as a gallium source in CVD. The conventional method of producing trimethylgallium is described by D.F. Gaines et al., Inorganic Synthesis 15 (1974) 203-207. Gallium trichloride is reacted with TMA according to the following reaction: EQU GaCl.sub.3 +3(CH.sub.3).sub.3 A1.fwdarw.Ga(CH.sub.3).sub.3 +3(CH.sub.3).sub.2 AlCl.
Synthesis of TMG and disposal of the residual material is described as follows:
"Sodium fluoride is dried in vacuo overnight. The system (FIG. 7) [The FIGURE in this application], containing 10 g. of sodium fluoride in flask D, is flushed with dry nitrogen for at least one hour. A glass ampul containing 25 g. of gallium trichloride is placed inside a small plastic bag which has a wall thickness of at least 0.002 in. (Alternatively two bags, one inside the other, afford added protection against hydrolysis.) The bag is purged with dry nitrogen, and the ampul is subsequently crushed with a hammer. Care should be taken that the broken glass does not puncture the bag (in practice this gave little trouble). The plastic bag is placed in a large nitrogen-filled glove bag and its contents, including broken glass, are emptied into the reaction flask B. The flask is attached quickly to the rest of the reaction apparatus, and about 80 ml. of trimethylaluminum (a twofold excess) is transferred from a lecture cylinder to the addition funnel. This transfer is accomplished most readily by inverting the lecture bottle, attaching a hose connector (to which a 4-in. length of ca. 1/8-in.-o.d. stainless-steel tubing is soldered), and cautiously opening the main lecture-bottle valve after the stainless-steel tube has been inserted into the addition funnel.
The trimethylaluminum is added slowly to the gallium trichloride over a period of about one hour. The reaction is extremely exothermic and is controlled by varying the addition rate of trimethylaluminum. External cooling should be avoided, as this may adversely affect the product yield. Initially, a few drops at a time are added; later, the rate may be increased. The flask contents are stirred magnetically during the reaction and distillation.
Immediately after the addition of trimethylaluminum is completed, external heating is begun and the crude product is distilled at 55-60.degree. onto the sodium fluoride in flask D. The pure product is redistilled at about 56.degree. from sodium fluoride into the receiving flask F. This flask is taken from the system, quickly fitted with an adapter equipped with a stopcock, and attached to a vacuum line for transfer into a storage vessel. (Caution. Some flaming of the product may occur when the receiving flask is removed from the apparatus; however, rapid and careful work should prevent any serious problems.) Extreme caution must be exercised in the disposal of the pyrophoric residues in flask B.
The pyrophoric residues in flask B may be safely disposed of using the following procedure. After flask B has cooled to room temperature, about 900 ml. of heptane is run into it through the additional funnel A. The mixture is then stirred magnetically to ensure that all soluble residues are dissolved. The resulting 10% solution in heptane does not appear to be pyrophoric (it does smoke, however, when exposed to air). Flask B is then removed from the rest of the apparatus and its contents slowly poured onto 2-3 lb. of crushed Dry Ice contained in a 5-gal metal bucket. After the dry ice-heptane solution slurry has stood for about 0.5 hour, 1 1. of 95% ethyl alcohol is added slowly to ensure complete solvolysis. The bucket is then left undisturbed until it has warmed to room temperature.
An alternate method suggested by the checkers is as follows. After flask B has cooled to room temperature about 600 ml. of a 10% ethyl acetate solution in heptane is run slowly into it through the addition funnel A. This addition is followed by slow addition of 300 ml. of 30% ethyl acetate in heptane. The mixture is stirred magnetically until all visible reaction has ceased. The resulting solution is cautiously poured onto 5 lb. of crushed ice in a 5-gal. metal bucket to complete the hydrolysis.
Care should be taken to dilute any remaining residues with heptane as the apparatus is being disassembled for cleaning. Any solid residues remaining in the apparatus should be covered with heptane and deactivated by slow addition of pentyl alcohol until there is no further evidence of reaction."
In accordance with the invention, it is found that the waste product of the conventional synthesis of TMG, rather that being discarded, can be used as a source of producing DMAH of high purity. The resulting DMAH is easily separated from the residual materials.